1. Field of the Invention
The invention relates to a field-effect-controlled semiconductor device having at least four regions of alternating opposite conduction type, in which regions are configured an anode-end emitter region, a first and second base region adjoining this anode-end emitter region, a cathode-end emitter region and a further adjacently disposed emitter region, wherein the two last-mentioned emitter regions form source and drain of a MOS field-effect transistor having an anode contact, a contact at the cathode-end emitter region and a control electrode contact of the MOS field-effect transistor.
A field-effect-controlled semiconductor device of the above-described type is known from U.S. Pat. No. 4,847,671. Such a semiconductor device mentioned in the patent specification "Emitter Switched Thyristor" (EST) is illustrated in the FIG. 5 and 6 and will be explained in greater detail in the following text. The field-effect-controlled semiconductor device, whose structure is illustrated in the FIG. 5 and 6, comprises an anode-end emitter region 10, a first base region 20, adjoining this anode-end emitter region followed by two second base regions 34, 36 and two cathode-end emitter regions 40, 44. On an insulating layer 50, which covers a part of the cathode-end base region 30, there is disposed a control electrode contact 60 called a gate, which forms a field-effect transistor with the cathode-end emitters 40, 44 and a channel region 42, 43. The device is provided with two power supply connections, a cathode 72 and an anode 74. Two thyristor structures are recognizable in the device described. The first, parasitic thyristor structure comprises the cathode-end emitter region 40, the two base regions 32, 20 adjoining this cathode-end emitter region and the anode-end emitter region 10 and may not be ignited in any operating condition. The second thyristor structure with the other cathode-end emitter structure 44, the two base regions 36, 20 adjoining this cathode-end emitter structure and the anode-end emitter region 10 form the main current path in the switched-on state.
The cathode-end emitter 40 is short-circuited with the cathode-end base region 34 via the cathode contact 72. In order to configure this shunt to be low-ohmic, the base region 30 is highly doped in a partial region 32. The main thyristor structure 44, 36, 20, 10 is controlled by a field-effect transistor 40, 50, 60, 44 and channel region 42, 43.
In the one embodiment of the known semiconductor device illustrated in FIG. 5, the doping in a moderately doped partial base region 34 determines the threshold voltage of the field-effect transistor and the injection efficiency of the cathode-end emitter 44. If the semiconductor device is polarized in forward direction and if the gate connection 60 of the field-effect transistor is actuated with a positive potential vis-a-vis the cathode, a conductive channel 42 forms in the p-base region 34, this channel connecting the two cathode-end emitters 40, 44 at a low resistance.
Simultaneously, a conductive channel 46 forms between the emitter 44 and the first base region 20. The electron current thus created acts as control current for an anode-end p-n-p transistor and offers the gate trigger current or hold current for the main thyristor 44, 34, 20, 10. The hole current flowing off toward the cathode contact 72 via the partial region 34 of the base region 30 polarizes the n.sup.+ -emitter 44 in forward direction and the injected electrons reinforce the conductance modulation of the moderately doped n-base region 20.
The regenerative actuation of the thyristor can be interrupted by equating the gate potential with the cathode potential so that the n-conducting channel of the field effect transistor disappears and the electron current is interrupted.
This process leads to the switching-off of the semiconductor component. The component structure according to FIG. 5 must be very carefully optimized, because the n.sup.+ -emitter of the parasitic thyristor structure 40, 30, 20, 10 is partially embedded in the same partial region 34 of the base region 30 as the emitter 44 but may not be polarized in forward direction in any operating condition. This limitation increases the gate trigger current and limits the maximum current of the component that can be switched off.
A more advantageous known embodiment is shown in FIG. 6. In the structure according to FIG. 6, the cathode-end base region 34 of FIG. 5 is divided into two partial regions 34, 36, wherein the partial regions 34, 36 are separated by an n-doped region 22 of the first n-base region 20.
The highly-doped partial region 32 provides a low-ohmic shunt of the anode-end emitter 40. The other partial region 34 defines the threshold voltage of the channel region 42, and the field-effect transistor 40, 42, 22 provides the gate trigger current for the main thyristor. The independently fabricated partial region 36 can be optimized with respect to the injection efficiency of the n.sup.+ -emitter 44 and the resistance of a shunt. This shunt of the emitter 44 is implemented via a resistance coupling of the partial region 36 to the cathode contact 72. The main thyristor structure 44, 36, 20, 10 is connected to the cathode 72 via a conductive channel 42, 22, 43 of the field-effect transistor.
An equivalent circuit diagram of the semiconductor device according to FIG. 6 is illustrated in FIG. 7. The separation of the partial regions 32, 36 facilitates the layout of shorting resistors R.sub.1 and R.sub.2. The resistance of resistor R.sub.1 should be kept as low as possible, and the value of resistor R.sub.2 must be optimized with respect to the switching characteristics of the device. With regard to the optimization, however, a compromise between the forward voltage and the maximum current that can be switched off must be accepted.
It is also known to connect the first base region 36 adjacent to the emitter region 44 of the main thyristor via a diode D with the cathode. The depletion of charge carriers is easily achieved when the device is switched off.
FIG. 1 schematically illustrates the structure of a field-effect-controlled semiconductor device, combining some known features, having a highly p-doped emitter layer 10 at the anode-end, with a first base region 20 comprising an n-doped region, with a second base region that is provided with a partial region 32 having a high p-doping, and a partial region 34 with p-doping as well as a further partial region 36 with p-doping and having highly n-doped emitter regions 40, 44 at the cathode-end.
The semiconductor device is provided with an anode connection A of an anode contact 74, a cathode connection K of a cathode contact 72 and a gate connection G of a gate 60. An insulating layer 50 separates the gate 60 from the base regions 20, 22, 34, 36 and from the cathode-end emitter regions 40, 44. The partial regions 34, 36 of the second base region are separated from each other by an n-doped region 22 of the first base region 20.
The semiconductor device contains a main thyristor structure having the cathode-end emitter region 44, the first and second base region 20, 34, 36 and the anode-end emitter layer 10. The main thyristor structure is controlled by means of a field-effect transistor comprising the cathode-end emitter regions 40, 44, the insulating layer 50 and the gate 60.
The cathode contact 72 forms an ohmic contact both with the emitter region 40 and with the partial region 32 of the second base region. The second base region is divided into the partial regions 32, 34 and 36. It is obvious that, to this extent, the semiconductor device of FIG. 1 corresponds to the known semiconductor device illustrated in FIG. 6.
In the semiconductor structure illustrated in FIG. 1, a highly p-doped region 38, which is disposed adjacent to the cathode-end base region adjoining the emitter region 44 of the main thyristor, is connected to the cathode contact 72 via an integrated component having a non-linear current/voltage characteristic.
In the highly-doped partial region 32 of the second base region a highly n-doped region 48 is provided, which is connected to a contact 76 via an ohmic contact and a metallic line which is configured as a track and, in contrast to the illustration according to FIG. 1, extends in the third dimension, the contact being adjacent to the region 38. The p-n junction between the n.sup.+ -region 48 and the p.sup.+ -region 32 acts in the manner of a Zener diode polarized in reverse direction, the diode being connected in the third dimension to the partial region 36, 38 of the base region via the contacts 76, 78 and the metal track.
The partial region 38 was provided to improve the ohmic metal/semiconductor junction 76, 38, but it may be omitted, because a Schottky contact would not impede the operation of the component at this point. Therefore, the partial region 36 may also be connected directly to the contact 78 by means of a contact and a metallic line.
But it is also possible that, in the structure illustrated in FIG. 5, the partial region 34, corresponding to the region 38 according to FIG. 1, is connected to a highly n-doped region in the partial region 32 and is thus linked to the cathode contact 72 via a contact, a metallic conductor and a further contact, with the cathode contact then not extending over the entire partial region 32.